Piezoelectricity basically is an energy transformation phenomenon between mechanical energy and electrical energy. As there is a cause and effect relationship between electric charge and strain, for an object which can generate piezoelectricity, as long as its crystal structure can generate electron dipole moment (EDM) when subject to an external force, transformation between the mechanical energy and electrical energy occurs. Then piezoelectricity can take place.
Generation of EDM is caused by a special arrangement of the positive ions and negative ions inside the crystal. Different crystal lattice systems in the material can generate different EDM. The basic condition is the asymmetrical center of the crystal lattice. If there is a symmetrical center in the crystal lattice, the positive ions and the negative ions will be neutralized. Then EDM cannot be generated. And transformation between the mechanical energy and electrical energy does not take place. Transformation between the mechanical energy and electrical energy generally has three forms: (1) transforming mechanical energy to kinetic energy, (2) transforming electrical energy to mechanical energy, and (3) transforming electrical energy to mechanical energy, then to electrical energy again to be output. Piezoelectric ceramics can transform mechanical energy to electrical energy and vice versus, thus is widely used. It may be used to fabricate pressure detection elements, accelerators, micro-motors and the like. The piezoelectric film can meet the requirements of thin and light that are demanded by many modern electronic products, hence it may be used in electronic elements for bio-detection and communication and the like. Its application potentials are highly appreciated in academics and industries.
Refer to FIGS. 1A and 1B for a conventional piezoelectric structure. Its structure is based on a principle announced by Berlincourt in 1973 named “unipoled PZT”. The relationship of thickness of the polarized areas of the piezoelectric structure that are corresponding to the input/output areas is changeable. The generated polarization process can alter the step-up ratio and step-down ratio of the piezoelectric structure. This is the principle of the structure. However, in the conventional techniques, changing the polarized areas of the input electrode and the output electrode by fixing the distance between the input electrode/output electrode and the ground electrode of the piezoelectric structure (namely di=do) to alter the step-up ratio and step-down ratio. This causes a problem which is if there is a desire to alter the step-up or step-down ratio, the polarized areas of the electrode and output electrode must be changed. Hence the size of the piezoelectric structure has to be increased or reduced. Its industrial applicability is greatly affected by the fabrication techniques and installation space of the piezoelectric structure.
U.S. Pat. No. 6,707,235 discloses an annular piezoelectric structure which has a first portion and a second portion. These two portions connect to an AC power supply. And other portions can produce and transform vibration generated by piezoelectric structure. The polarized direction of the annular piezoelectric structure is normal to the perimeter surface of the annular body. It generates different step-up and step-down ratios through different shapes of the piezoelectric structure. The different shapes of the piezoelectric structure create difficulty in manufacturing.